When AI Gets It Wrong: Hallucinations, Fake Law and Out-of-Date Figures

You ask an AI assistant about probate. Back comes an answer in seconds: clear, confident and better written than most letters from professionals. It sounds right. That is the problem. Fluency and accuracy are different things, and nothing on the screen tells you which you have.

This is not a lecture aimed at people who should have known better. In June 2025 the High Court dealt with two cases where invented law reached a courtroom, one involving a barrister. In December 2025 the Court of Appeal called resort to AI by people without a lawyer "entirely understandable". If this has caught you out, you are in a large group, some of it wearing a wig.

Estate planning is unusually exposed, because so much of it rests on numbers that keep moving. Our companion factsheet, Using AI to Make Sense of Legal Jargon, covers what these tools do well. This one covers where they go wrong.

Why a confident answer can still be wrong

A language model predicts the next word. It has no filing cabinet to look things up in. It gives you the most likely continuation of your question: usually right, occasionally invented with total confidence.

Why guess rather than admit doubt? Because the way these systems are scored rewards guessing, like a multiple choice exam. Guess and you might get lucky. Leave it blank and you score nothing. It is also why the spelling is never wrong but the facts sometimes are: grammar follows patterns the model has seen endlessly, and a one-off fact, such as one fee on one date, does not.

Fluency tells you nothing about accuracy either. Longer, more authoritative-sounding answers hallucinate more, because they contain more claims that can be false.

Failure mode one: sources that do not exist

The anchor case is R (Ayinde) v London Borough of Haringey; Al-Haroun v Qatar National Bank [2025] EWHC 1383 (Admin), a Divisional Court decision of 6 June 2025 before the President of the King's Bench Division. In Ayinde a barrister cited five non-existent cases, one tell-tale sign being Americanised spelling ("emphasized"). In Al-Haroun, 45 citations were put before the court and 18 did not exist. They came from the client's own AI use, adopted unchecked by his solicitor. Both were referred to their regulators.

Nor was it the first:

• Harber v HMRC [2023] UKFTT 1007 (TC), 4 December 2023. A litigant in person cited nine non-existent tribunal decisions. The first UK instance.
• Bandla v SRA [2025] EWHC 1167 (Admin), 13 May 2025. Twenty-seven authorities the other side could not locate, never checked. Indemnity costs of £24,727.20.
• Cork v Smith [2026] EWHC 1199 (Ch), 22 May 2026. A fabricated statute: an invented text of an Insolvency Rule quoted as though real. A major law firm, a partner and a senior associate publicly admonished.
• D (A Child) (Recusal) [2025] EWCA Civ 1570, 9 December 2025. A mother representing herself cited cases that did not exist, and the Court of Appeal called that entirely understandable.

An academic database of AI hallucination cases had recorded 1,846 worldwide as at 4 August 2026, 61 in the UK, roughly double six months earlier. People representing themselves (1,080) outnumber lawyers (717). Those most likely to be caught out are ordinary people trying to help themselves.

How often does it happen?

• A 2024 Stanford study of general-purpose AI on US case law: 58% to 88%.
• A 2025 study of specialist legal tools built for lawyers: still 17% to 33%.
• A November 2025 study of real public legal advice questions: 55% for London.
• October 2025, the European Broadcasting Union and BBC, four search-enabled assistants, 3,000-plus responses: 45% had at least one significant issue, 20% had major accuracy problems including out-of-date information.

Searching the web helps. It does not fix it.

Failure mode two: American law in English clothes

Far more of the world's legal writing online is American than English, so a question about wills often drifts into US law. It reads like law. It is just the wrong country's law. The swaps to watch for:

• "Living trust." Not a thing here in the same way. We have will trusts, life interest trusts and discretionary trusts, all registrable on HMRC's Trust Registration Service.
• "Probate court." There is no separate probate court. Applications go to HM Courts and Tribunals Service, disputes to the High Court, Chancery Division.
• "Estate tax" and a multi-million dollar exemption. Here it is Inheritance Tax: nil rate band £325,000, residence nil rate band £175,000, rate 40%. That imported exemption figure does the most damage, because it persuades people they have nothing to plan for. See Inheritance Tax Mitigation.
• Community property. Does not exist here. Property is held as joint tenants or tenants in common, and survivorship on a joint tenancy operates outside your will.
• "Pour-over will." No equivalent, and no purpose.
• "Power of attorney" in the American sense. Here it is a Lasting Power of Attorney, in two types, each registered with the Office of the Public Guardian. An unregistered LPA cannot be used.
• "Advance healthcare directive" or "living will." Here it is an Advance Decision to Refuse Treatment under the Mental Capacity Act 2005.
• "Notarise your will." Wills are not notarised here. Two witnesses must be present at the same time, and a witness who benefits loses the gift.
• "Die intestate and your spouse gets everything." Only if there are no children. With children the spouse takes the personal belongings, a statutory legacy of £322,000 and half the residue. See Intestacy.

Failure mode three: figures that were right once

This is the quietest failure, and in estate planning the most common. Every model has a date beyond which it knows nothing, and it does not tell you. The boundary is fuzzy too: one vendor's own documentation says its model's cutoff is March 2026, but that in some subjects its knowledge runs only to January 2025. A 14-month spread inside one model, admitted by its makers.

The current position, and what a stale answer says:

• Probate. The application fee is £526 from 13 July 2026, not £300, and estates of £5,000 or under pay nothing. Sealed copies are £2 each with the application and £16 each later, not £1.50. GOV.UK says applications are usually dealt with within 12 weeks, not 16.
• Lasting Powers of Attorney. Registration is £92 per LPA, £184 for both, not £82 and £164, and £46 for a repeat within three months. It takes 8 to 10 weeks, not up to 20, and the notice period is three weeks, not four.
• Fee reductions. The Office of the Public Guardian gives 50% off where the donor's gross income is under £12,000. Universal Credit is now assessed for a reduction only, not an automatic exemption (form LPA120, revised 2 February 2026), and fees are not refundable retrospectively.
• Inheritance Tax. The nil rate band is £325,000 and the residence nil rate band is £175,000, and both are frozen to 5 April 2031. With a home passing to children that is £500,000, or £1 million for a couple, tapering away above £2 million.
• Pensions. Unused pensions come into Inheritance Tax for deaths on or after 6 April 2027, under the Finance Act 2026. An older answer says pensions sit outside the estate.
• Intestacy. The statutory legacy is £322,000, unchanged since 26 July 2023, and not an annual figure. A stale answer says £270,000.
• Agricultural and Business Relief. A combined £2.5 million 100% allowance from 6 April 2026, 50% relief above that, transferable between spouses. Many answers still say £1 million, the original 2024 proposal before it was raised in December 2025.

One caveat. Both bands are frozen to 5 April 2031, under a single provision: Finance Act 2026 section 72. Yet HMRC's own threshold table still shows the residence nil rate band ending on 5 April 2030. That is a mistake on the page, not a different rule for the second band; one table was updated and the other missed. So a reader who checks the official page currently gets the wrong date. It does not change the advice. GOV.UK is still a far better guide than any AI's memory, and right almost all of the time. It is also written by people, and people occasionally miss a row. Where a figure matters the legislation is the last word, and this is a moment to ask a professional rather than rely on any single page.

A worked example

Imagine Dan, 61, making Lasting Powers of Attorney with his wife while handling his late father's estate administration. He asks an AI assistant sensible questions and gets confident answers: LPA registration is £82 each and takes up to 20 weeks; because he receives Universal Credit the fee is waived automatically and anything he pays he can reclaim later; probate is £300, copies £1.50. So he sends both LPAs off without payment and puts £300 aside.

Every one of those answers was true once. None is true now. Registration is £92 per LPA, £184 for both, and takes 8 to 10 weeks. Universal Credit is assessed for a reduction, not an automatic exemption, so the applications stall over an unpaid fee, with nothing to reclaim afterwards. Probate is £526 and copies are £2 each. Five minutes on GOV.UK would have caught all of it. This is a hypothetical example for illustration only.

The five-minute check

1. Ask which GOV.UK page it got that from, then click the link. Three warning signs: the link is dead; it works but the figure is not on the page; or it is not gov.uk, legislation.gov.uk or judiciary.uk.
2. Check the figure on the authoritative page yourself. Probate fees: gov.uk/applying-for-probate/fees. Probate timescales: gov.uk/applying-for-probate/after-youve-applied. LPA fees and timescale: gov.uk/power-of-attorney/register. LPA fee reductions: gov.uk/government/publications/power-of-attorney-fees. Inheritance Tax: gov.uk/inheritance-tax. Residence nil rate band: gov.uk/inheritance-tax/passing-on-home. Intestacy: gov.uk/inherits-someone-dies-without-will.
3. Check the "Last updated" date at the bottom. Every GOV.UK page carries one.
4. Check that a case exists. Paste the citation into Find Case Law at caselaw.nationalarchives.gov.uk or BAILII at bailii.org. Nothing back means treat it as fiction.
5. When AI and GOV.UK disagree, GOV.UK wins. Every time.

What AI cannot answer at all

Some questions are not about finding a better model or prompt. No AI can answer:

• anything needing your own documents, which raises its own questions, covered in Should You Upload Your Will or Trust Deed to ChatGPT?;
• anything about the current state of your own estate;
• anything needing judgement about your family;
• anything turning on facts only you hold;
• anything needing a witness or a capacity assessment.

The safe framing: AI is reasonable at what a term means, unreliable on what a number currently is, and cannot tell you what you should do. If you are an executor wondering what to do first, our Executors: The First 30 Days factsheet is a better place to start.

Common mistakes

Treating confidence as evidence, and citations as facts. The tone of an answer tells you nothing about whether it is right. A case name and a court take a moment to invent and thirty seconds to verify on Find Case Law. Barristers have skipped that step and been referred to their regulators.

Acting on a figure without checking its date. Fees and thresholds here change more often than people expect, and the "Last updated" line settles it in seconds.

Letting a bad experience stop you asking questions. Confident bad information is not new, and not unique to computers. Frightening subjects have always attracted it, which is why we wrote Care Fees and Your Home: Myths vs Facts. Ask the questions. Just check the answers.

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Figures and case references above were checked against GOV.UK, legislation.gov.uk, HMRC and Find Case Law in August 2026. Fees change, so check the current position on GOV.UK before you act. This factsheet is general information for England and Wales, not legal, tax or financial advice. Please seek personal advice about your own circumstances. Last reviewed: August 2026.